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   Re: Locating schemas (was Re: Irony heaped on irony)

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  • From: ht@cogsci.ed.ac.uk (Henry S. Thompson)
  • To: John Aldridge <john.aldridge@informatix.co.uk>
  • Date: 22 May 2000 17:03:06 +0100

John Aldridge <john.aldridge@informatix.co.uk> writes:

> [to dereference, or not to dereference, NSURI or xsi:schemaLocation]

> And I disagree with both of you :-)
> The problem with dereferencing either the namespace URI or the
> schemaLocation attribute is that it means the _author_ of the XML
> document is providing explicit instructions on how to locate the
> schema.
> Unfortunately, this author may have no understanding of the
> environment of the _reader_ of the XML document. The schema location
> used by the XML author may be inaccessible to the reader, and this
> reader should not have to resort to editing the XML document before
> (s)he can make sense of it.
> I don't object to URI dereferencing as a fallback if the reader has
> not given more explicit instructions, but the primary method should be
> for the reader to "install" a schema locally, and for XML processing
> software then to "know" where to find the schema information for the
> corresponding namespace.

Indeed this is precisely what the current XML Schema WD allows, see 
section 6.3.2 How schema definitions are located on the Web [1]:

   Given a namespace URI (or none) and (optionally) a URI reference
   from xsi:schemaLocation or xsi:noNamespaceSchemaLocation,
   schema-aware processors may implement any combination of the
   following strategies, in any order:

     1.1 Do nothing, for instance because a schema containing
     components for the given namespace URI is already known to be
     available, or because it is known in advance that no efforts to
     locate schema documents will be successful (for example in
     embedded systems);

     1.2 Based on the location URI, identify an existing schema
     document, either as a text/xml resource or a schema element
     information item, in some local schema repository;

     1.3 Based on the namespace URI, identify an existing schema
     document, either as a text/xml resource or a schema element
     information item, in some local schema repository;

     1.4 Attempt to resolve the location URI, to locate a resource on
     the web which is or contains or references a schema element;

     1.5 Attempt to resolve the namespace URI to locate such a
     resource.  Whenever possible configuration and/or invocation
     options for selecting and/or ordering the implemented strategies
     should be provided.


[1] http://www.w3.org/TR/xmlschema-1/structures.html#schema-loc
  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2001, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/

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